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I have two domains, one forwarded to the other. I want to use 2 e-mail accounts on both domains as aliased but some e-mails only on the other domain. I will create 2 MX records for both domains with different priorities.

I wonder that if a mailserver checks domain MX and couldn't find the e-mail account on MX 10 priority server, will it skip to check MX 80 priority server or return "mailbox couldn't find" error?

My MX scenario is as below:

firstdomain.com MX 10 (with only 2 mailbox) seconddomain.com MX 80 (with 2 mailbox and other mailboxes)

I hope I could explaing it clear. Thank you!

Update for theCleaner =================================

The story is: domainA.com: New domain domainB.com: Older domain

My client has an old domain domainB.com and redirected it to domainA.com after purchasing domainA.com. He has many email accounts on domainB.com. Also his john@domainB.com mailbox is active and known by his clients. He wants to buy Exchange mailbox from me. He wants john@domainA.com on Exchange server but also wants john@domainB.com as an Accepted-Domain on Exchange. He will use only john@domainA.com and john@domainB.com on Exchange server. His employees will continue to use emails on domainB.com

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  • These would be 2 servers for the same domain? MX priorities are used in that sense, MX records for domain.com IN MX 10 mail.domain.com IN MX 40 backup.domain.com the priorities are only recommendations, nothing says you can't send to the second priority, and obviously, they mail servers do not have to have the same domain name as the domain they are the mailserver for (but it isn't a bad idea). – NickW May 30 '13 at 11:16
  • Yes 2 server for same domain. 2 user will use Exchange mailbox for 2 domains, one of it will be aliased. The other users will use standart mailboxes on second domain. 2 Exchange users want to receive emails for both domains, second domain as Accepted-Domain. – Kemal May 30 '13 at 11:21
  • I think that this isn't the situation you want to use MX priorities on, as in those situations, either server can be chosen. – NickW May 30 '13 at 11:22
  • Thank you NickW The best solution is trying I guess. Let's see if another solution advised. – Kemal May 30 '13 at 11:34
  • you tagged this with "Exchange". Are you using Exchange? Two different Exchange organizations or one? Maybe you can update your question with a real world example like "bob@domainA.com, etc." Just hoping to see what you are really trying to accomplish in the end. – TheCleaner May 30 '13 at 13:05
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I'm not sure I've completely understood your question, but I'll try and answer.

MX priorities are there more for high availability or backup purposes, if two servers are going to be receiving mail for a single domain, they need to have the exact same users and aliases, though they could have different destinations, the primary server is where the mailboxes are, so the secondary server will accept mail then forward it to the primary. The priorities are there so that if one of the servers is unavailable, mail can still be accepted for that domain.

If a server responds, either priority 10 or priority 80, and says "550 mailbox not found" then that is the definitive answer for that email, and any functional mail server will then return the email to sender. Creating two mail servers for a domain means that both servers must be able to accept mail for any user configured @yourdomain.com.

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  • Are you sure about sender's server won't lookup priority 80 server if it has answer 'not found' from priority 10? – Kemal May 30 '13 at 11:44
  • Pretty sure, if it returns a 550 no, if you can configure both servers to return a 551, then it might, but that's not something I'd honestly bet any money on. From the RFCs, Servers MAY reject or bounce messages when they are not deliverable when addressed. When they do so, they MAY either provide address-updating information with a 551 code, or may reject the message as undeliverable with a 550 code and no address-specific information. But, if a 551 code is used, they MUST NOT assume that the client will actually update address information or even return that information to the user. – NickW May 30 '13 at 11:54

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